Method for making carbon nanotubes

ABSTRACT

A method of making carbon nanotubes is provided, the method includes depositing a catalyst layer on a substrate, placing the substrate having the catalyst layer in a reaction furnace, heating the reaction furnace to a predetermined temperature, introducing a carbon source gas and a protective gas into the reaction furnace to grow a first carbon nanotube segment structure comprising a plurality of metallic carbon nanotube segments, and applying a pulsed electric field to grow a second carbon nanotube segment structure from the plurality of metallic carbon nanotube segments, where the pulsed electric field is a periodic electric field including a plurality of positive electric field pulses and a plurality of negative electric field pulses alternately arranged, and the second carbon nanotube segment structure includes a plurality of semiconducting carbon nanotube segments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/979,423, filed on May 14, 2018, entitled,“METHOD FOR MAKING CARBON NANOTUBES”, which claims all benefits accruingunder 35 U.S.C. § 119 from China Patent Application No. 201710349116.X,filed on May 17, 2017, in the China National Intellectual PropertyAdministration, the contents of which are hereby incorporated byreference. The disclosures of the above-identified applications areincorporated herein by reference.

FIELD

The subject matter herein generally relates to a method for makingcarbon nanotubes.

BACKGROUND

Carbon nanotubes have excellent properties such as electricalproperties, high Young's modulus and tensile strength, and high thermalconductivity.

In carbon nanotubes prepared by the conventional chemical vapordeposition (CVD) method, the proportion of metallic carbon nanotubes(m-CNTs) and semiconducting carbon nanotubes (s-CNTs) is typically about1:2.

It has been impossible to directly grow s-CNTs with higher purity.Meanwhile, the chirality of carbon nanotubes cannot be arbitrarilychanged during growth, in the prior art. Semiconducting carbon nanotubesegments and metallic carbon nanotube segments cannot be alternatelyformed during growth according to need. Thus, carbon nanotubes withalternating semiconducting carbon nanotube segments and metallic carbonnanotube segments have not been available for carbon nanotube structuresor in the application of carbon nanotube structures, such as thin filmtransistors, light detectors, and photoelectric conversion modules.

Therefore, there is room for improvement within the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views. Implementations of the present technologywill be described, by way of example only, with reference to theattached figures

FIG. 1 is a flow chart of an embodiment of a method for making carbonnanotubes.

FIG. 2 is a schematic view of an embodiment of an electric field tocontrol the chirality of carbon nanotubes.

FIG. 3 is a schematic view of an embodiment of controlling the chiralityof carbon nanotubes using an electric field.

FIG. 4 is a schematic view of an embodiment of carbon nanotubes withchirality after removing the electric field.

FIG. 5 is a Scanning Electron Microscope (SEM) image of carbon nanotubesbefore and after applying a reverse electric field.

FIG. 6 is a schematic view of an embodiment of controlling the formationof carbon nanotubes with chirality using an electric field.

FIG. 7 is a schematic view of an embodiment of the chirality of carbonnanotubes after removing the electric field.

FIG. 8 is a schematic view of an embodiment of a pulsed electric fieldvoltage changing with time.

FIG. 9 is a schematic view of an embodiment of a chiral change of carbonnanotubes in a pulsed electric field when a pulse width is less than 500ms.

FIG. 10 is a schematic view of an embodiment of a method of removing ahead portion of a carbon nanotube structure.

FIG. 11 is a schematic view of an embodiment of the chirality of carbonnanotubes changing with electric field when the pulse width is greaterthan 500 ms.

FIG. 12 is a schematic view of an embodiment of the chirality of newcarbon nanotubes changing with a pulsed electric field.

FIG. 13 is a schematic view of an embodiment of the chirality of newcarbon nanotubes changing after removing the pulsed electric field.

FIG. 14 is a schematic view of an embodiment of the chirality of newcarbon nanotubes changing after removing the pulsed electric field.

FIG. 15 is an SEM image of carbon nanotubes when the pulse width is 100ms.

FIG. 16 is an SEM image of carbon nanotubes when the pulse width is 10seconds.

FIGS. 17(a)-(e) are structural schematic views of an embodiment of acarbon nanotube array.

FIG. 18 is a structural schematic view of an embodiment of a thin filmtransistor.

FIG. 19 is a chart showing test results in an embodiment of the thinfilm transistor.

FIG. 20 is a structural schematic view of another embodiment of a thinfilm transistor.

FIG. 21 is a structural schematic view of an embodiment of a lightdetector.

FIG. 22 is a structural schematic view of an embodiment of aphotoelectric conversion device.

FIG. 23 is a sectional schematic view of another embodiment of thephotoelectric conversion device.

FIG. 24 is a structural schematic view of another embodiment of aphotoelectric conversion device.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth to provide a thoroughunderstanding of the embodiments described herein. However, it will beunderstood by those of ordinary skill in the art that the embodimentsdescribed herein can be practiced without these specific details. Inother instances, methods, procedures, and components have not beendescribed in detail so as not to obscure the related relevant featurebeing described. The drawings are not necessarily to scale, and theproportions of certain parts may be exaggerated to better illustratedetails and features. The description is not to be considered aslimiting the scope of the embodiments described herein.

Several definitions that apply throughout this disclosure will now bepresented.

The connection can be such that the objects are permanently connected orreleasably connected. The term “substantially” is defined to beessentially conforming to the particular dimension, shape, or otherfeature that the term modifies, such that the component need not beexact. The term “comprising” means “including, but not necessarilylimited to”; it specifically indicates open-ended inclusion ormembership in a so-described combination, group, series, and the like.It should be noted that references to “an” or “one” embodiment in thisdisclosure are not necessarily to the same embodiment, and suchreferences mean “at least one.”

In FIG. 1, an embodiment of a method of making carbon nanotubescomprises:

-   -   S11, depositing a catalyst layer 12 on a substrate 11;    -   S12, placing the substrate 11 having the deposited catalyst        layer 12 in a reaction furnace 13, then heating the reaction        furnace 13 to obtain a predetermined temperature, introducing a        carbon source gas 14 and a protective gas 15 into the reaction        furnace 13 to grow a first carbon nanotube structure 161, where        the first carbon nanotube structure 161 includes a plurality of        carbon nanotube segments;    -   S13, applying an electric field to the first carbon nanotube        structure 161, where a direction of the electric field is the        direction in which the catalyst layer 12 is positively charged;        and    -   S14, reversing the direction of the electric field to grow a        second carbon nanotube structure 162 from the first carbon        nanotube structure 161, where the first carbon nanotube        structure 161 and the second carbon nanotube structure 162 form        a carbon nanotube structure 16, and the second carbon nanotube        structure 162 includes a plurality of semiconducting carbon        nanotube segments.

In step S11, carbon nanotubes are grown on the substrate 11. Thematerial of the substrate 11 can be silicon, glass, quartz, or the like.In an embodiment, the substrate 11 is ST-cut single crystal quartzsubstrate.

The catalyst layer 12 can be a catalyst powder layer. The material ofthe catalyst layer 12 can be any catalyst material sufficient to growsingle-walled carbon nanotubes. The catalyst layer 12 can be an ironpowder layer, an iron film, a nickel powder layer, a nickel mesh, or amixture layer of alumina powder and iron powder. A method of forming thecatalyst layer 12 on the substrate 11 can be electron beam deposition,vapor deposition, sputtering, or spraying. In an embodiment, thecatalyst layer 12 is an iron powder layer with a particle diameter of0.2 nanometers, the catalyst layer 12 is deposited on the substrate 11by electron beam deposition.

In step S12, the substrate 11 having the deposited catalyst layer 12 isplaced in the reaction furnace 13. The predetermined temperature is in arange of 800 degrees Celsius to 960 degrees Celsius. The predeterminedtemperature can be in a range of 900 degrees Celsius to 950 degreesCelsius. In an embodiment, the predetermined temperature is 950 degreesCelsius.

The carbon source gas 14 can be carbon monoxide, or hydrocarbon such asacetylene, methane, ethane, or ethylene, or vapor containing carbonatoms, like ethanol. The protective gas 15 can be hydrogen gas, nitrogengas, or another inert gas. Flow ratios of the carbon source gas 14 andthe protective gas 15 can be adjusted according to the type andrequirement of the carbon source gas. In an embodiment, the carbonsource gas 14 is methane, and the protective gas 15 is hydrogen gas. TheH₂ with a flow rate of 500 sccm and CH₄ with a flow rate of 200 sccm areintroduced into the reaction furnace 13 for ten minutes, then the H₂with a flow rate of 5 sccm and CH₄ with a flow rate of 2 sccm areintroduced into the reaction furnace 13 to grow the first carbonnanotube structure 161.

The method of growing the first carbon nanotube structure 161 is notlimited to the growth method in steps S11-S12 described above, themethod can be arc discharge method or laser ablation method. The firstcarbon nanotube structure 161 includes a plurality of carbon nanotubesegments. The plurality of carbon nanotube segments is arranged in anarray, and the plurality of carbon nanotube segments substantiallyextends along a same direction. The quantity density of the carbonnanotube segments in the first carbon nanotube structure 161 is greaterthan 3/μm to ensure that the chirality of carbon nanotube segments canbe effectively controlled by the electric field in the growth process ofthe carbon nanotube segments. The term “quantity density” refers to thenumber of carbon nanotube segments per micron along a direction which isperpendicular to the growth direction of the carbon nanotube segments.In an embodiment, the carbon nanotube segments are arranged in ahorizontal array, and the quantity density of the carbon nanotubesegments is greater than 6/μm. The growth process of the plurality ofcarbon nanotube segments can be bottom growth, or top growth. When thegrowth process of the plurality of carbon nanotube segments is bottomgrowth, the catalyst layer 12 is adhered to a surface of the substrate11, and the carbon nanotube segments start growing from a surface of thecatalyst layer 12 and grow away from the catalyst layer 12. When thegrowth process of the plurality of carbon nanotube segments is topgrowth, the carbon nanotube segments start growing from the surface ofthe catalyst layer 12 and grow with the catalyst layer 12. The firstcarbon nanotube structure 161 includes a plurality of metallic carbonnanotube segments. The carbon nanotube segments of the first carbonnanotube structure 161 are single-walled carbon nanotubes. A diameter ofa single-walled carbon nanotube is less than 2 nanometers. The diameterof the single-walled carbon nanotube can be in a range of 1.2 nanometersto 1.5 nanometers. In an embodiment, the first carbon nanotube structure161 includes both semiconducting carbon nanotube segments and metalliccarbon nanotube segments, and the ratio of the semiconducting carbonnanotube segments and the metallic carbon nanotube segments is about orless than 2:1.

In step S13, the electric field applied to the carbon nanotube structure16 can be a direct current (DC) electric field or a pulsed electricfield. In FIG. 2, a direction of the electric field is set to a positivedirection when the catalyst layer 12 is positively charged. A directionof the electric field is set to a negative direction when the catalystlayer 12 is negatively charged. The positive direction can be reversedto the negative direction. In FIG. 3, “s” refers to semiconducting, “m”refers to metallic, and “E” refers to the applied electric field. In anembodiment, the electric field is the DC electric field. When a positiveelectric field is first applied to the first carbon nanotube structure161, the growth of the first carbon nanotube structure 161 is notaffected and the chirality of the carbon nanotube segments in the firstcarbon nanotube structure 161 remains unchanged. Thus, the metalliccarbon nanotube segments remain as pure metallic carbon nanotubesegments before and after applying the positive electric field, and thesemiconducting carbon nanotube segments remain as pure semiconductingcarbon nanotube segments before and after applying the positive electricfield.

In step S14, the positive electric field is reversed to a negativeelectric field. Such change from the positive electric field to thenegative electric field causes the chirality of the grown carbonnanotube segments to change. When an electric field direction changesfrom the positive direction to the negative direction, the carbonnanotube segments of the second carbon nanotube structure 162 are mostlysemiconducting carbon nanotube segments. The second carbon nanotubestructure 162, grown from the semiconducting carbon nanotube segments ofthe first carbon nanotube structure 161, are semiconducting carbonnanotube segments. The carbon nanotube segments of the second carbonnanotube structure 162, that are grown from the metallic carbon nanotubesegments of the first carbon nanotube structure 161, are alsosemiconducting carbon nanotube segments.

The second carbon nanotube structure 162 can become pure semiconductingcarbon nanotube segments by applying an inverted electric field. Thechange of the electric field direction from positive to negativeincreases negative charges, which increases the Fermi level of thecatalyst layer 12. The higher the Fermi level, the easier is thetransition from metallic carbon nanotube segments to semiconductingcarbon nanotube segments. In an embodiment, a raised value of the Fermilevel of the catalyst layer 12 is greater than or equal to 0.7 eV. In anembodiment, the raised value of the Fermi level of the catalyst layer 12is 1 eV.

When the positive electric field is originally applied to the firstcarbon nanotube structure 161, an application time of the positiveelectric field must not be too short, the carbon nanotube segments musthave enough time inside a positive electric field environment. In anembodiment, the application time of the positive electric field isgreater than or equal to 2 seconds, and the electric field intensity ofthe positive electric field is greater than or equal to 200 volts permillimeter (v/mm). When the positive electric field is reversed, theelectric field environment of the carbon nanotube segments changesviolently, which directly affects the chirality of the carbon nanotubesegments. Gibbs free energy for growing semiconducting carbon nanotubesis less than Gibbs free energy for growing metallic carbon nanotubesafter the catalyst layer 12 is negatively charged. When the electricfield reverses from positive to negative, the second carbon nanotubestructure 162 mostly comprises semiconducting carbon nanotube segments.The proportion of semiconducting carbon nanotube segments in the secondcarbon nanotube structure 162 is related to an application time of thenegative electric field. The application time of the negative electricfield depends on the area of electrodes. The larger the area ofelectrodes, the longer the application time of the negative electricfield. For example, when the area of electrodes is very small (4 mm×8mm), the carbon nanotube segments can become completely semiconductingcarbon nanotube segments with the application time of the negativeelectric field being 100 ms; when the area of electrodes is very large(4 cm×4 cm), the carbon nanotube segments can become completelysemiconducting carbon nanotube segments with the application time of thenegative electric field being 500 ms, the carbon nanotube segments canbecome completely semiconducting carbon nanotube segments when theapplication time of the negative electric field is in a range of 100 msto 500 ms. In an embodiment, the area of electrodes is 5 mm×2 cm, andthe electrodes are used in the following. When the application time ofthe negative electric field is less than 200 ms, only part of the carbonnanotube segments of the second carbon nanotube structure 162 aresemiconducting carbon nanotube segments. If the application time of thenegative electric field is too short, other part of the carbon nanotubesegments of the second carbon nanotube structure 162 remains metalliccarbon nanotube segments. When the application time of the negativeelectric field is greater than 200 ms, all the carbon nanotube segmentsof the second carbon nanotube structure 162 become semiconducting carbonnanotube segments. In FIG. 4, when all the carbon nanotube segments ofthe second carbon nanotube structure 162 are semiconducting carbonnanotube segments, the growth of the carbon nanotube structure 16 is notaffected by the negative electric field. After removing the negativeelectric field, the chirality of new carbon nanotube segments grown fromthe carbon nanotube structure 16 will remain unchanged. In FIG. 5 is anSEM image of carbon nanotubes before and after applying the reverseelectric field. The white strips are metallic carbon nanotube segments,the dark strips are semiconducting carbon nanotube segments. In anembodiment, the application time of the positive electric field is 20seconds, the electric field intensity of the positive electric field is200 v/mm, the application time of the negative electric field is 500milliseconds, and the electric field intensity of the negative electricfield is −200 v/mm.

Alternatively, in FIG. 6, when the electric field direction changes fromnegative to positive, the chirality of the carbon nanotube segments canalso change during growth. When the negative electric field is firstapplied to the first carbon nanotube structure 161, the growth of thefirst carbon nanotube structure 161 is not affected and the chirality ofthe carbon nanotube segments in the first carbon nanotube structure 161remains unchanged. When the electric field direction changes fromnegative to positive, the second carbon nanotube structure 162 aremostly metallic carbon nanotube segments. The second carbon nanotubestructure 162, grown from the metallic carbon nanotube segments of thefirst carbon nanotube structure 161, are metallic carbon nanotubesegments. The second carbon nanotube structure 162, grown from thesemiconducting carbon nanotube segments of the first carbon nanotubestructure 161, are also metallic carbon nanotube segments. The change ofthe electric field direction from negative to positive increasespositive charges, which can reduce the Fermi level of the catalystlayer. The lower the Fermi level, the easier is the transition fromsemiconducting carbon nanotube segments to metallic carbon nanotubesegments. In an embodiment, the reduced value of the Fermi level of thecatalyst layer 12 is less than or equal to −0.1 eV. In an embodiment,the reduced value of the Fermi level of the catalyst layer 12 is lessthan or equal to −0.2 eV. In FIG. 7, when all the carbon nanotubesegments of the second carbon nanotube structure 162 are metallic carbonnanotube segments, the growth of the carbon nanotube structure 16 is notaffected by the positive electric field. After removing the positiveelectric field, the chirality of new carbon nanotube segments grown fromthe carbon nanotube structure 16 remains unchanged.

Furthermore, applying a pulsed electric field to the first carbonnanotube structure 161 can change the chirality to an alternatingchirality. In FIG. 8, the pulsed electric field is a periodic electricfield formed by a positive electric field pulse followed by a negativeelectric field pulse. A period T of the pulsed electric field is a sumof a pulse width of one positive electric field pulse and a pulse widthof one negative electric field pulse. The chirality of the carbonnanotube segments in the carbon nanotube structure 16 can be controlledby adjusting the pulse width of the negative electric field pulse andthe electric field direction.

In FIG. 8, when the pulse width of the negative electric field pulse isless than 200 milliseconds, and the electric field direction changesfrom positive to negative, only part of the carbon nanotube segments ofthe second carbon nanotube structure 162 are semiconducting carbonnanotube segments. The change in the field direction (from negative topositive), does not change the chirality of the carbon nanotube segmentsof the second carbon nanotube structure 162.

In FIG. 9, when the pulse width of the negative electric field pulse isgreater than 200 milliseconds and less than or equal to 500milliseconds, and the electric field direction changes from positive tonegative, all the carbon nanotube segments of the second carbon nanotubestructure 162 are semiconducting carbon nanotube segments. The change inthe field direction (from negative to positive), does not change thechirality of the carbon nanotube segments of the second carbon nanotubestructure 162.

Furthermore, in FIG. 10, pure semiconducting carbon nanotubes can beobtained by removing the metallic carbon nanotube segments of the carbonnanotube structure 16, where the metallic carbon nanotube segments arein a head portion of the carbon nanotube structure 16.

In FIG. 11, when the pulse width of the negative electric field pulse isgreater than 500 milliseconds, and the electric field direction changesfrom positive to negative, all the carbon nanotube segments of thesecond carbon nanotube structure 162 are semiconducting carbon nanotubesegments. When the electric field direction changes from negative topositive, the carbon nanotube segments of the second carbon nanotubestructure 162 can become metallic carbon nanotube segments. When thepulse width of the negative electric field pulse is too long, thecatalyst layer 12 can spontaneously discharge so that the negativecharge decreases. Upon the electric field direction changing fromnegative to positive, the positive electric field pulse applied to thecatalyst layer 12 is sufficient to change the chirality fromsemiconducting to metallic. When the pulse width of the negativeelectric field pulse is greater than 500 milliseconds, as theapplication time of the pulsed electric field increases, the chiralityof carbon nanotube segments which are growing from the carbon nanotubestructure 16 changes alternately.

In FIG. 12, as the period T increases, new carbon nanotube segmentsgrown from the carbon nanotube structure 16 consist of alternatingmetallic carbon nanotube segments and semiconducting carbon nanotubesegments.

Furthermore, in FIG. 13 and FIG. 14, after removing the pulsed electricfield, the chirality of the new carbon nanotube segments remainsunchanged. Pure metallic carbon nanotubes can be obtained by removingthe head portion of the carbon nanotube structure 16.

In FIG. 15, an SEM image of carbon nanotube structure is shown, wherethe period T of the pulsed electric field is 20 seconds, and the pulsewidth of the negative electric field pulse is 100 milliseconds. Thebright white strips are metallic carbon nanotube segments, and the lightwhite strips are semiconducting carbon nanotube segments.

In FIG. 16, an SEM image of carbon nanotube structure is shown, wherethe period T of the pulsed electric field is 20 seconds, and the pulsewidth of the negative electric field pulse is 10 seconds. The carbonnanotube structure is an array formed by a plurality of carbonnanotubes, where each of the carbon nanotubes is forming semiconductingcarbon nanotube segments and metallic carbon nanotube segments on analternating basis.

The method of making carbon nanotubes includes particular advantages.Firstly, the chirality of carbon nanotube segments grown from a carbonnanotube structure can be adjusted by applying an electric field whichis reversible. Secondly, alternating semiconducting carbon nanotubesegments and metallic carbon nanotube segments can be obtained byadjusting a pulse width of a negative electric field pulse and anelectric field direction.

In FIGS. 17(a)-(e), an embodiment of a carbon nanotube array 20 made bythe above method comprises a plurality of carbon nanotubes. Each of thecarbon nanotubes includes at least one semiconducting carbon nanotubesegment (S) and at least one metallic carbon nanotube segment (M). Theplurality of carbon nanotubes is arranged to form an array. Thesemiconducting carbon nanotube segment (S) and the adjacent metalliccarbon nanotube segment (M) are connected by the Schottky barrier.

The plurality of carbon nanotubes comprises semiconducting carbonnanotube segments and metallic carbon nanotube segments. Each of thecarbon nanotubes can consist of only one semiconducting carbon nanotubesegment and only one metallic carbon nanotube segment, and thesemiconducting carbon nanotube segment and the metallic carbon nanotubesegment are an integrated structure, and connected by the Schottkybarrier. Each of the carbon nanotubes can also consist of a plurality ofsemiconducting carbon nanotube segments and a plurality of metalliccarbon nanotube segments, and each of the semiconducting carbon nanotubesegments and each of the metallic carbon nanotube segments arealternately arranged. The semiconducting carbon nanotube segments andthe adjacent metallic carbon nanotube segments are connected by theSchottky barrier.

In FIGS. 17(a)-(e), in the carbon nanotube array 20, the structure ofthe carbon nanotubes can be M-S-M type carbon nanotubes, S-M-S typecarbon nanotubes, or S-M type carbon nanotubes. In the plurality ofcarbon nanotubes, the lengths of the semiconducting carbon nanotubesegments are substantially the same, and the lengths of the metalliccarbon nanotube segments are substantially the same. The term“substantially” means that the lengths of the carbon nanotube segmentsmay be slightly different in each carbon nanotube. In the actualproduction process, since the growth rate of each carbon nanotube isslightly different, the length of each carbon nanotube may be slightlydifferent at the point when the direction of the electric field changes.The length of each semiconducting carbon nanotube segment or the lengthof each metallic carbon nanotube segment in the carbon nanotubes can beadjusted by changing the application time period of the electric field.The carbon nanotubes in the carbon nanotube array 20 are single-walledcarbon nanotubes, and a diameter of the single-walled carbon nanotubesis less than 2 nanometers. Furthermore, the diameter of thesingle-walled carbon nanotubes is in a range of 1.2 nanometers to 1.5nanometers. In an embodiment, each of the carbon nanotubes consists of aplurality of semiconducting carbon nanotube segments and a plurality ofmetallic carbon nanotube segments. The semiconducting carbon nanotubesegments and the metallic carbon nanotube segments are alternatelyarranged, and the diameter of each of the carbon nanotubes is 1.3nanometers.

The carbon nanotube array 20 has particular advantages. Each of thecarbon nanotubes includes metallic carbon nanotube segments andsemiconducting carbon nanotube segments. The carbon nanotube array 20 isan integrated structure being partly conductive and partlysemiconductive. Thus, the carbon nanotube array 20 has a wide range ofpotential uses.

For example, in FIG. 18, an embodiment of a thin film transistor 30comprises an insulating substrate 31, a gate electrode 32, a gateinsulating layer 33, a carbon nanotube structure 34, a source electrode35, and a drain electrode 36. The carbon nanotube structure 34 includesat least one carbon nanotube. One end of the carbon nanotube is a firstmetallic carbon nanotube segment 341, and the other end of the carbonnanotube is a second metallic carbon nanotube segment 342. There is asemiconducting carbon nanotube segment 343 in the middle of the carbonnanotube. The gate electrode 32 is located on a surface of theinsulating substrate 31. The gate insulating layer 33 is located on asurface of the gate electrode 32 away from the insulating substrate 31.The carbon nanotube structure 34 is located on a surface of the gateinsulating layer 33 away from the gate electrode 32. The sourceelectrode 35 is located on the first metallic carbon nanotube segment341, and the drain electrode 36 is located on the second metallic carbonnanotube segment 342. The source electrode 35 and the drain electrode 36are electrically connected to the carbon nanotube structure 34. Thesemiconducting carbon nanotube segment 343 is used as a channel. Thecarbon nanotube structure 34, the source electrode 35, and the drainelectrode 36 are insulated from the gate electrode 32. Furthermore, thesource electrode 35 and the drain electrode 36 are selectable, thus thefirst metallic carbon nanotube segment 341 can be used as a sourceelectrode, and the second metallic carbon nanotube segment 342 can beused as a drain electrode.

The insulating substrate 31 is a support. Materials of the insulatingsubstrate 31 can be rigid materials (e.g., glass, quartz, ceramics,diamond), or flexible materials (e.g., plastic or resin). The insulatingsubstrate 31 can be polyethylene terephthalate, polyethylenenaphthalate, or polyimide. In an embodiment, the material of theinsulating substrate 31 is polyethylene naphthalate.

The gate electrode 32, the source electrode 35, and the drain electrode36 are conductive materials. The conductive materials can be metal,indium tin oxide, antimony tin oxide, conductive silver paste,conductive polymer, and conductive carbon nanotubes. The material of themetal can be aluminum, copper, tungsten, molybdenum, gold, titanium, orpalladium. In an embodiment, the gate electrode 32, the source electrode35, and the drain electrode 36 are metal composite structures formed bygold and titanium, the gold being located on a surface of the titanium.

The material of the gate insulating layer 33 can be rigid materials,such as alumina, yttrium oxide, silicon nitride, or silicon oxide. Thematerial of the gate insulating layer 33 can also be flexible materials,such as benzocyclobutene, polyester, or acrylic resin. A thickness ofthe gate insulating layer 33 is in a range of 0.5 nanometers to 100micrometers. In an embodiment, the material of the gate insulating layer33 is alumina, and the thickness of the gate insulating layer 33 is 40nanometers.

The carbon nanotube structure 34 includes at least one carbon nanotube.When the carbon nanotube structure 34 includes a plurality of carbonnanotubes, the plurality of carbon nanotubes is combined tightly by theVan der Waals force to form a carbon nanotube film. The plurality ofcarbon nanotubes extends substantially along a same direction. Each ofthe carbon nanotubes consists of two metallic carbon nanotube segmentsand a semiconducting carbon nanotube segment in the middle of the carbonnanotube. The lengths of the metallic carbon nanotube segments ofadjacent carbon nanotubes are substantially the same, and the lengths ofthe semiconducting carbon nanotube segments of adjacent carbon nanotubesare substantially the same. The metallic carbon nanotube segments of theplurality of carbon nanotubes are the ends of the carbon nanotubes. Themetallic carbon nanotube segments at one end of the carbon nanotubestructure 34 are in direct contact with the source electrode 35, and themetallic carbon nanotube segments at the other end of the carbonnanotube structure 34 are in direct contact with the drain electrode 36.Thus, the source electrode 35 and the drain electrode 36 have a goodelectrical connection within the carbon nanotube structure 34. The M-S-Mtype of the carbon nanotube structure 34 can be obtained by the abovemethod. The lengths of the channel and the metallic carbon nanotubesegments 341, 342 can be adjusted by controlling the electric field.

In use, the source electrode 35 is grounded. A voltage Vg is applied onthe gate electrode 32. Another voltage Vd is applied to the drainelectrode 36. The voltage Vg forms an electric field in the channel ofthe carbon nanotube structure 34. Accordingly, carriers exist in thechannel near the gate electrode 32. As the voltage Vg increases, acurrent is generated and flows through the channel. Thus, the sourceelectrode 35 and the drain electrode 36 are electrically connected.

In FIG. 19, a chart of test results of an embodiment of the thin filmtransistor 30 is shown. The carbon nanotube structure 34 of the thinfilm transistor 30 consists of forty carbon nanotubes. A switching ratioof the thin film transistor 30 can be 2×10⁵:1.

The thin film transistor 30 has following advantages. The metalliccarbon nanotube segments 341, 342 of the carbon nanotubes structure 34can be used as source electrodes and drain electrodes. Thus, thestructure of the thin film transistor is simple and there is no need toprepare extra electrodes. Since the source electrode, the drainelectrode, and the semiconductor layer are integrated, the interfacebetween the semiconductor layer and the source electrode, and the drainelectrode is reduced as a barrier, and the switching ratio is increased.When an independent metal source electrode and an independent metaldrain electrode are provided, both ends of the carbon nanotubes beingmetallic carbon nanotube segments, the metallic carbon nanotube segmentshave excellent electrical connection with the independent metal sourceelectrode and the independent metal drain electrode.

In FIG. 20, an embodiment of a thin film transistor 40 comprises aninsulating substrate 41, a carbon nanotube structure 42, a sourceelectrode 43, a drain electrode 44, an insulating layer 45, and a gateelectrode 46. The carbon nanotube structure 42 includes at least onecarbon nanotube. One end of the carbon nanotube is a first metalliccarbon nanotube segment 421, and the other end of the carbon nanotube isa second metallic carbon nanotube segment 422. A semiconducting carbonnanotube segment 423 is in the middle of the carbon nanotube. Thesemiconducting carbon nanotube segment 423 is used as a channel. Thecarbon nanotube structure 42 is located on a surface of the insulatingsubstrate 41. The source electrode 43 is in direct contact with thefirst metallic carbon nanotube segment 421, and the drain electrode 44is in direct contact with the second metallic carbon nanotube segment422. The source electrode 43 and the drain electrode 44 are electricallyconnected to the carbon nanotube structure 42. The insulating layer 45is located on a surface of the carbon nanotube structure 42. The gateelectrode 46 is located on a surface of the insulating layer 45.

The thin film transistor 40 is similar to the thin film transistor 30except that the thin film transistor 40 is a top gate type thin filmtransistor. The insulating layer 45 insulates the gate electrode 46 fromthe source electrode 43, the drain electrode 44, and the carbon nanotubestructure 42.

In FIG. 21, an embodiment of a light detector 50 comprises a carbonnanotube structure 51, a first electrode 52, a second electrode 53, anda current detection device 54. The carbon nanotube structure 51 iselectrically connected to the first electrode 52 and the secondelectrode 53. The current detection device 54, the first electrode 52,the second electrode 53, and the carbon nanotube structure 51 areelectrically connected to form a circuit in series. The carbon nanotubestructure 51 is the same as the carbon nanotube structure 34. Both endsof the carbon nanotube structure 51 are metallic carbon nanotubesegments, and the first electrode 52, the second electrode 53 arelocated on surfaces of the metallic carbon nanotube segments and indirect contact with the metallic carbon nanotube segments.

In the carbon nanotube structure 51, the M-S-M type of the carbonnanotube includes a heterojunction and is used to detect light.

The first electrode 52 and the second electrode 53 are both composed ofconductive materials. The conductive materials can be metal, indium tinoxide, antimony tin oxide, conductive silver paste, conductive polymer,and conductive carbon nanotubes. The material of the metal can bealuminum, copper, tungsten, molybdenum, gold, titanium, or palladium.The first electrode 52 and the second electrode 53 can also beconductive films. A thickness of the conductive films is in a range of 2microns to 100 microns. In an embodiment, the first electrode 52 and thesecond electrode 53 are metal composite structures formed of gold andtitanium. The titanium is located on the carbon nanotube structure 51,and the gold is located on the titanium. A thickness of titanium is 2nanometers, and a thickness of gold is 50 nanometers.

The current detection device 54 detects the current in the circuit. Thecurrent detection device 54 can be an ammeter. Furthermore, the lightdetector 50 includes a power source 55, the power source 55 can providea bias voltage between the first electrode 52 and the second electrode53.

Furthermore, the light detector 50 includes a substrate 56, thesubstrate 56 supporting the carbon nanotube structure 51. When thecarbon nanotube structure 51 is a free-standing structure, the substrate56 can be omitted. The term “free-standing structure” means that thecarbon nanotube structure can sustain the weight of itself when hoistedby a portion thereof without any significant damage to its structuralintegrity. The material of the substrate 56 can be insulating material,such as glass, ceramic, polymer, or wood material. The material of thesubstrate 56 can also be conductive metal material coated with aninsulating material. In an embodiment, the material of the substrate 56can be glass.

The light detector 50 can detect light. The working process of the lightdetector 50 comprises turning on the power source 55, and applying avoltage between the first electrode 52 and the second electrode 53. Inthe absence of light, no photo-generated carriers are produced in thecarbon nanotube structure 51, the heterojunction is on off-status, andno current passes through the series circuit. No current change isdetected by the current detection device 54. If light irradiates thecarbon nanotube structure 51, the photo-generated carriers are producedin the carbon nanotube structure 51, the heterojunction is on on-status,a current passes through the series circuit, and the current detectiondevice 54 detects a current change.

The light detector 50 can be used quantitatively as well asqualitatively. For quantitative function, the light detector 50 turns onthe power source 55 and applies a voltage between the first electrode 52and the second electrode 53. As light of different strengths irradiatesthe carbon nanotube structure 51, different current values correspondingto the lights with different strengths are recorded, and a graph aboutthe light strengths and the current values can be drawn. If light withunknown strength irradiates the carbon nanotube structure 51, a currentvalue corresponding to such light can be detected, and according to thegraph about the light strengths and the current values, the strength ofthe light can be calculated.

The light detector 50 has following advantages. Since a detection pointof the light detector 50 is the carbon nanotube structure containing aheterojunction, the heterojunction is formed by the semiconductingcarbon nanotube segments and the metallic carbon nanotube segments. Thesemiconducting carbon nanotube segments and the metallic carbon nanotubesegments are integrated as a structure, which increases ease ofconduction and sensitivity to current. Therefore, the light detector 50has a simple structure and high sensitivity.

In FIG. 22 and FIG. 23, an embodiment of a photoelectric conversiondevice 60 comprises a photoelectric conversion module 61, a coverstructure 62, and a substrate 63. The photoelectric conversion module 61is located on the substrate 63. The photoelectric conversion module 61includes a carbon nanotube structure 64. The carbon nanotube structure64 is the same as the carbon nanotube structure 34. One end of thecarbon nanotube structure 64 is a first metallic carbon nanotube segment641, the other end of the carbon nanotube structure 64 is a secondmetallic carbon nanotube segment 642, and a semiconducting carbonnanotube segment 643 is in the middle of the carbon nanotube. Thesemiconducting carbon nanotube segment 643 includes a covered area 643 band a non-covered area 643 a. The cover structure 62 covers the coveredarea 643 b of the photoelectric conversion module 61.

FIG. 22 shows a top view of the photoelectric conversion device 60. FIG.23 shows a sectional schematic view of the photoelectric conversiondevice 60.

The substrate 63 supports the photoelectric conversion module 61. Whenthe photoelectric conversion module 61 is a free-standing structure, thesubstrate 63 can be omitted. The material of the substrate 63 can beinsulating material, such as glass, ceramic, polymer, or wood material.The material of the substrate 63 can also be conductive metal materialcoated with an insulating material. The material of the substrate 63should not be infrared-absorbent. A thickness of the substrate 63 is notlimited. The thickness of the substrate 63 is in a range of 1 millimeterto 2 centimeters. In an embodiment, the material of the substrate 63 isglass, and the thickness of the substrate 63 is 5 millimeters.

The semiconducting carbon nanotube segment 643 is divided into thecovered area 643 b and the non-covered area 643 a. The areas of thecovered area 643 b and the non-covered area 643 a is not limited. Thearea of the covered area 643 b can be greater than, equal to, or lessthan the area of the non-covered area 643 a. In an embodiment, the areaof the covered area 643 b is equal to the area of the non-covered area643 a.

The non-covered area 643 a receives light energy and converts the lightenergy into heat energy, raising the temperature of the non-covered area643 a. Thus, a temperature difference is generated between the coveredarea 643 b and the non-covered area 643 a. A potential difference isgenerated in the semiconducting carbon nanotube segment 643 by thethermoelectric effect. The light energy can be from sunlight, visiblelight, infrared or ultraviolet, or even electromagnetic waves outsidethe visible spectrum.

The photoelectric conversion module 61 comprises a first electrode 65and a second electrode 66. The first electrode 65 is electricallyconnected to the first metallic carbon nanotube segment 641, and thesecond electrode 66 is electrically connected to the second metalliccarbon nanotube segment 642. The first electrode 65 and the secondelectrode 66 are voltage outputs of the photoelectric conversion device60. Furthermore, the chirality of the first metallic carbon nanotubesegment 641 and the second metallic carbon nanotube segment 642 aremetallic. Thus, the first metallic carbon nanotube segment 641 and thesecond metallic carbon nanotube segment 642 can be independentelectrodes, or the first metallic carbon nanotube segment 641 and thesecond metallic carbon nanotube segment 642 can be used as voltageoutputs.

The photoelectric conversion device 60 further includes a first leadwire (not shown) and a second lead wire (not shown). The first lead wireis electrically connected to the first electrode 65, the second leadwire is electrically connected to the second electrode 66. The firstlead wire can facilitate a connection to the first electrode 65 and thesecond lead wire can facilitate a connection to the second electrode 66,without a circuit.

The cover structure 62 is used to cover the covered area 643 b of thephotoelectric conversion module 61 to prevent light-irradiation of thecovered area 643 b. The cover structure 62 should not cover thenon-covered area 643 a. The material of the cover structure 62 can beconductive or insulating. The conductive material can be metal or alloy,such as stainless steel, carbon steel, copper, nickel, titanium, zinc,or aluminum. The insulating material can be resin or plastic. When thecover structure 62 is insulating material, the cover structure 62 can bein direct contact with the covered area 643 b and cover the covered area643 b. When the cover structure 62 is conductive material, the coverstructure 62 should be spaced apart and insulated from the covered area643 b. In an embodiment, the cover structure 62 is a housing with anaccommodating space, and the cover structure 62 is fixed on thesubstrate 63. The covered area 643 b is located inside the accommodatingspace of the cover structure 62. The covered area 643 b is spaced fromthe cover structure 62. When the material of the substrate 63 and thematerial of the cover structure 62 are both insulating materials, thecover structure 62 and the substrate 63 can form an integratedstructure.

The photoelectric conversion device 60 has following advantages. Whenthe non-covered area 643 a is exposed to light, a temperature differenceis generated between the covered area 643 b and the non-covered area 643a, electricity being generated by the thermoelectric effect. Thesemiconducting carbon nanotube segment 643, the first metallic carbonnanotube segment 641, and the second metallic carbon nanotube segment642 are integrated, reducing the resistance of the interface as abarrier and increasing the output power.

In FIG. 24, an embodiment of a photoelectric conversion device 70comprises a photoelectric conversion module 71, a cover structure 72,and the substrate 73. The photoelectric conversion module 71 is locatedon the substrate 73. The photoelectric conversion module 71 includes acarbon nanotube structure 74. The carbon nanotube structure 74 comprisesa plurality of metallic carbon nanotube segments and a plurality ofsemiconducting carbon nanotube segments alternately arranged.

The photoelectric conversion device 70 is similar to the photoelectricconversion device 60 except that the photoelectric conversion device 70comprises the plurality of metallic carbon nanotube segments and theplurality of semiconducting carbon nanotube segments alternatelyarranged. Since the metallic carbon nanotube segments can be used aselectrodes, the photoelectric conversion device 70 includes theplurality of semiconducting carbon nanotube segments connected inseries. Thus, the output power of the photoelectric conversion device 70is increased. The cover structure 72 includes a plurality of spacedcovering bodies 721 and a plurality of spaced openings 722. Each of theplurality of semiconducting carbon nanotube segments has a first portioncovered by one of the plurality of spaced covering bodies 721 and asecond portion exposed from one of the plurality of spaced openings 722.The spaced openings can be windows using transparent materials, such asglass.

The embodiments shown and described above are only examples. Even thoughnumerous characteristics and advantages of the present technology havebeen set forth in the foregoing description, together with details ofthe structure and function of the present disclosure, the disclosure isillustrative only, and changes may be made in the detail, including inmatters of shape, size, and arrangement of the parts within theprinciples of the present disclosure, up to and including the fullextent established by the broad general meaning of the terms used in theclaims.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may comprisesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion for ordering the steps.

What is claimed is:
 1. A method of making carbon nanotubes, the methodcomprising: depositing a catalyst layer on a substrate; placing thesubstrate having the catalyst layer thereon in a reaction furnace;heating the reaction furnace to a predetermined temperature; introducinga carbon source gas and a protective gas into the reaction furnace togrow a first carbon nanotube segment structure comprising a plurality offirst metallic carbon nanotube segments; applying a pulsed electricfield to grow a second carbon nanotube segment structure from theplurality of first metallic carbon nanotube segments, wherein the pulsedelectric field is a periodic electric field comprising a plurality ofpositive electric field pulses and a plurality of negative electricfield pulses alternately arranged, the second carbon nanotube segmentstructure comprises a plurality of semiconducting carbon nanotubesegments, a direction of the plurality of positive electric field pulsesis along a first direction in which the catalyst layer is positivelycharged, and a direction of the plurality of negative electric fieldpulses is along a second direction in which the catalyst layer isnegatively charged.
 2. The method as claimed in claim 1, wherein theplurality of first metallic carbon nanotube segments extends along asame direction.
 3. The method as claimed in claim 1, wherein theplurality of first metallic carbon nanotube segments are single-walledcarbon nanotube segments, and a diameter of the single-walled carbonnanotube segments is less than 2 nanometers.
 4. The method as claimed inclaim 3, wherein the diameter of the single-walled carbon nanotubesegments is in a range of 1.2 nanometers to 1.5 nanometers.
 5. Themethod as claimed in claim 1, wherein a quantity density of theplurality of first metallic carbon nanotube segments is greater than3/μm.
 6. The method as claimed in claim 1, wherein a quantity density ofthe plurality of first metallic carbon nanotube segments is greater than6/μm.
 7. The method as claimed in claim 1, wherein a Fermi level of thecatalyst layer is raised by the plurality of positive electric fieldpulses to obtain a raised value, and the raised value is greater than orequal to 0.7 eV.
 8. The method as claimed in claim 1, wherein a firstpulse width of the negative electric field pulse is less than 200 ms,and only part of the second carbon nanotube segment structure aresemiconducting carbon nanotube segments.
 9. The method as claimed inclaim 1, wherein a second pulse width of the negative electric fieldpulse is greater than 200 ms, and less than or equal to 500 ms, and allthe second carbon nanotube segment structure are semiconducting carbonnanotube segments.
 10. The method as claimed in claim 1, wherein a thirdpulse width of the negative electric field pulse is greater than 500 ms,the second carbon nanotube segment structure comprises a plurality ofsecond metallic carbon nanotube segments and the plurality ofsemiconducting carbon nanotube segments alternately arranged.
 11. Amethod of making carbon nanotubes, the method comprising: growing afirst carbon nanotube segment structure from a catalyst layer; applyingan electric field to the first carbon nanotube segment structure tocharge the catalyst layer; and reversing a direction of the electricfield to grow a second carbon nanotube segment structure from the firstcarbon nanotube segment structure, wherein the first carbon nanotubesegment structure and the second carbon nanotube segment structure havedifferent chirality, the direction of the electric field is reversedfrom a positive direction to a negative direction, and the second carbonnanotube segment structure are semiconducting carbon nanotube segments;the direction of the electric field is reversed from the negativedirection to the positive direction, and the second carbon nanotubesegment structure are metallic carbon nanotube segments; and thepositive direction is in which the catalyst layer is positively charged,and the negative direction is in which the catalyst layer is negativelycharged.